Hi everyone, I have a 2.5 gallon and it's been running for about 2 weeks now with a couple tiny fish.... The filter looks dirty already with green on it. I know the filter builds up good bacteria to eat away the ammonia and stuff, but is it bad that it's green? When do I know to change it? And also, when I do change it, how do I keep some of the good bacteria from the old filter in the water? If I just put a new filter on than I'm getting rid of the good bacteria. Here is a pic of what it looks like..